Methods and systems for hcn removal

ABSTRACT

Hydrogen cyanide removal during transformation of a carbon fiber precursor into carbon fiber is provided. The method may comprise heating a carbon fiber body precursor at a substantially atmospheric pressure to above about 1200° C. in a furnace in a first stage, expelling a stream of effluent gas outside the furnace, wherein the stream of effluent gas comprises a cyanide, thermally oxidizing the cyanide during the first stage, and heating the carbon fiber body precursor to a temperature of between about 1600° C. and about 2200° C. in a second stage.

FIELD OF INVENTION

The present invention generally relates to methods and systems for HCNremoval, and more particularly, to methods and systems for HCN removalduring transformation of a carbon fiber precursor into carbon fiber.

BACKGROUND OF THE INVENTION

Industrial applications of ceramics have become increasingly importantover the last fifty years. Monolithic ceramics and cermets, however,exhibit low impact resistance and low fracture toughness. Ceramic MatrixComposites (CMCs) exhibit some useful thermal and mechanical propertiesand hold the promise of being very good materials for use in hightemperature environments and/or in heat sink applications. CMCsgenerally comprise one or more ceramic materials disposed on or withinanother material, such as, for example, a ceramic material disposedwithin a structure comprised of a fibrous material. Fibrous materials,such as carbon fiber, may be formed into fibrous bodies suitable forthis purpose.

Carbon fiber bodies are typically formed from carbon fiber bodyprecursors. For example, preoxidized polyacrylonitrile (PAN) is commonlyused as a carbon fiber body precursor. Carbon fiber body precursors maybe manipulated and fabricated in a manner similar to a textile (e.g.,weaving, knitting, etc) to form desired structures. To transform thecarbon fiber body precursor into a carbon fiber body, various methodsand techniques may be used. For example, during transformation of PANmaterials, the PAN fiber may be carbonized and then processed toeliminate metals or other impurities that may be found in the PAN fiber.

Transformation of carbon fiber body precursors, such as PAN fibers,often occurs in a two stage process. The first stage may be acarbonization stage. A carbonization stage is typically performed attemperatures of less than 1100° C., and most typically between about800° C. and 950° C. The second stage may be a high temperature stage,typically using temperatures over 1400° C.

However, the carbonization causes the PAN fibers to release variouscyanides (such as HCN) in a gaseous state, and the subsequent managementof the residual impurities is often problematic. For example, cyanideslike HCN are toxic and pose an environmental hazard. Conventional meansof managing cyanides such as HCN are not satisfactory because cyanidesare often released along with other hazardous materials and/orproblematic gaseous materials. Moreover, it may be difficult to separateHCN from other materials for appropriate processing. Further still,where HCN is emitted in a second stage, HCN may be drawn into downstreamequipment, such as a steam vacuum. The presence of HCN in a steam vacuummay damage the steam vacuum and, in some case, endanger those in thevicinity. Conventional means make it difficult for cyanides to bereleased during a carbonization stage.

Accordingly, there is a need for managing cyanide release in acontrolled manner such that the cyanides can be managed appropriately.For example, there is a need to release HCN in a carbonization stage sothat HCN may exit a furnace system prior to the use of a vacuumgenerating device, such as a steam vacuum.

SUMMARY OF THE INVENTION

In various embodiments, methods and systems for HCN removal areprovided. For example, a method may include transforming a carbon fiberbody precursor into a carbon fiber body, comprising heating carbon fiberbody precursor at a substantially atmospheric pressure to above about1200° C. in a furnace in a first stage, expelling a stream of effluentgas outside the furnace, for example, by using the furnace's internalpressure, wherein the stream of effluent gas comprises a cyanide,thermally oxidizing the cyanide during the first stage, and heating thecarbon fiber body precursor to a temperature of between about 1600° C.and about 2200° C. in a second stage.

Various embodiments also include a system for transforming a carbonfiber body precursor into a carbon fiber body, comprising a furnacehaving a gas inlet, a thermal oxidizer, wherein the furnace isconfigured to heat a carbon fiber body precursor at a substantiallyatmospheric pressure to above about 1200° C. in a first stage, whereinthe furnace is configured to expel a stream of effluent gas outside thefurnace, wherein the stream of effluent gas comprises a cyanide, whereinthe thermal oxidizer is configured to thermally oxidize the cyanideduring the first stage to yield a less toxic material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an exemplary embodiment including a furnace;and

FIG. 2 is a flow chart illustrating an exemplary embodiment of a method.

DETAILED DESCRIPTION

The detailed description of exemplary embodiments herein makes referenceto the accompanying drawings, which show exemplary embodiments by way ofillustration and its best mode. While these exemplary embodiments aredescribed in sufficient detail to enable those skilled in the art topractice the invention, it should be understood that other embodimentsmay be realized and that logical, chemical and mechanical changes may bemade without departing from the spirit and scope of the invention. Thus,the detailed description herein is presented for purposes ofillustration only and not of limitation. For example, the steps recitedin any of the method or process descriptions may be executed in anyorder and are not necessarily limited to the order presented. Moreover,many of the functions or steps may be outsourced to or performed by oneor more third parties. Furthermore, any reference to singular includesplural embodiments, and any reference to more than one component or stepmay include a singular embodiment or step. Also, any reference toattached, fixed, connected or the like may include permanent, removable,temporary, partial, full and/or any other possible attachment option.Additionally, any reference to without contact (or similar phrases) mayalso include reduced contact or minimal contact.

As noted above, in various embodiments, methods and systems are providedherein that manage cyanide (e.g., HCN) in a carbon fiber productionprocess. In particular, systems and methods are disclosed herein thatimprove the separation of HCN from other materials in a gaseous streamso that HCN management may be improved. In this regard, it has beenfound that, contrary to conventional methods, use of a highercarbonization stage temperature assists in the separation andelimination of HCN from a gaseous stream of a carbon fiber productionprocess.

As used herein, a carbon fiber production process may be any processthat results in forming a carbon fiber body. Additionally, as usedherein, a carbon fiber body may comprise any material containing carbonfiber. As noted above, carbon fiber bodies are typically formed fromcarbon fiber body precursors in a carbon fiber transformation process.As used herein, a carbon fiber body precursor may comprise any materialcontaining a carbon fiber precursor. Carbon fiber precursors includepreoxidized polyacrylonitrile fiber, rayon fibers, and pitch fibers.

As noted above, to transform the carbon fiber body precursor into acarbon fiber body, various methods and techniques may be used. Forexample, during transformation of PAN materials, the PAN material may becarbonized and then processed to eliminate impurities (often comprisingmetals and metallic compounds) that may be found in the PAN material.HCN and/or other cyanides may be released from PAN fibers during carbonfiber precursor processing.

Transformation of carbon fiber body precursors, such as PAN fibers,often occurs in a two stage process in a process vessel, such as afurnace. The first stage may be referred to as a carbonization stage orcarbonization. A carbonization stage is conventionally performed attemperatures of less than about 1100° C., and most typically betweenabout 800° C. and about 950° C. Carbonization is typically performedfrom about atmospheric pressures (e.g., about 1 atm) to elevatedpressures (e.g., about 1.5 atm).

The second stage may be a high temperature stage, typically usingtemperatures over about 1400° C., up to and about 2400° C. The secondstage may be performed in a vacuum or partial vacuum. For example, apump or steam vacuum may be used to pull a vacuum in a furnace suitablefor the second stage. The second stage is referred to herein as a secondstage or a second stage of a carbon fiber transformation process.

During carbonization in stages such as those described above, HCN andother cyanides may enter a gas form and exit the furnace in a gaseousstream, usually by way of a pipe. During the second stage, HCN and othercyanides also may enter a gas form, often in higher concentration thanduring carbonization, and exit the furnace in a gaseous stream, usuallyby way of a pipe. As HCN and other cyanides are toxic and poseenvironmental hazards, carbonization processes benefit from systems andmethods to manage and/or eliminate HCN.

Conventional carbonization is performed below temperatures of about1100° C. because it is generally believed that temperatures above about1100° C. may cause an unwanted elimination of metallic impurities (e.g.,sodium) during carbonization, when it is desired that such impuritiesare not removed until the second stage. Conventionally, most, if notall, cyanides are released in the second stage. However, because thesecond stage occurs in a vacuum created by a vacuum device (e.g., pumpor steam vacuum), cyanides may become trapped in the vacuum device. Thismay damage the vacuum device and may pose a hazard for those near thedevice.

It has been found that, contrary to conventional teachings, performingcarbonization at temperatures above about 1100° C. allows HCN and othercyanides to be expelled from a furnace. The HCN and other cyanides maybe conducted to a thermal oxidizer and thermally oxidized (i.e., burned)to yield other, less or nontoxic materials, such as carbon dioxide,water, and nitrous oxides.

Further, it has been found that, in various embodiments, performingcarbonization at temperatures above about 1100° C. allows HCN to beseparated from other materials that are released by the carbon fiberprecursor bodies.

Still further, it has been found that, in various embodiments,performing carbonization at temperatures above about 1100° C. allows HCNto be removed in the carbonization stage. As the carbonization isperformed under substantially atmospheric conditions, HCN need not comeinto contact with a vacuum device and, instead, may be forwarded to athermal oxidizer, for example, by virtue of the internal pressure of thefurnace itself.

Accordingly, with reference now to FIG. 1, an embodiment of HCNelimination system 100 comprising a furnace 108 which is used to containand carbonize carbon fiber body precursors is illustrated. In variousembodiments, furnace 108 is any furnace capable of achieving thetemperatures used in the improved carbon fiber production process.Furnace 108 has inlet 102 and outlet 104. Inlet 102 and outlet 104 areconfigured to conduct gas in and out of furnace 108. Furnace 108 mayalso have seal flanges 110 that may be assembled (e.g., bolted) togetherto prevent unintentional gaseous emissions.

During conventional carbonization, non-cyanide impurities such ashydrocarbons may be released into gaseous form at temperatures belowabout 1100° C. At such temperatures, various emissions managementsystems are used to address these emissions. However, significantly, attemperatures above about 1100° C., such as at about 1200° C.,hydrocarbon emissions decrease. Thus, in various embodiments, whencarbonization is performed at about 1200° C., the gaseous emissions arecomprised substantially of HCN. For example, gaseous stream 121resulting from carbonization may contain HCN.

Thus, to address the HCN that is created, in various embodiments,carbonization is performed in furnace 108 at temperatures above about1100° C. For example, in various embodiments, carbonization may beperformed at temperatures from above about 1100° C. to about 1400° C.,from about 1200° C. and about 1300° C., and above about 1200° C.Carbonization may be performed in furnace 108 at pressures of from about1 atm to about 1.5 atm. For example, in various embodiments,carbonization may be performed at about 1 atm and at about 1.5 atm.Carbonization performed within these temperature and pressure ranges mayimprove the release of HCN and other cyanides from a furnace. Forexample, HCN and other cyanides may be expelled (i.e., automaticallyeliminated) from the furnace, for example, by virtue of the internalpressure of the furnace, requiring little to no pump or steam vacuumassistance. This is beneficial because pumps and/or steam vacuums may bedamaged by prolonged exposure to HCN, in addition to creating ahazardous environment for those in proximity to such equipment. In thisregard, such a process enhances the useful life of vacuum devices.

In this regard, HCN may be caused to flow out of a furnace via theinternal pressure of the furnace. In various embodiments, to assist HCNflow, a furnace may have an inlet for the acceptance of a secondary gaswhich may be pumped into a furnace inlet to push HCN and/or othercyanides out of the furnace. For example, with reference to FIG. 1, afurnace may have a secondary gas injected into it via inlet 102 toassist in the expulsion of gaseous materials from inside the furnace. Asecondary gas may comprise any substantially inert gas such as nitrogengas (N₂) and any noble gas such as helium, argon, neon, and krypton.

In various embodiments, carbonization may be performed at atmospheric orsubstantially atmospheric pressures. However, the pressure in furnace108 may be raised by, for example, a secondary gas pumped into inlet102.

In an embodiment, with continued reference to FIG. 1, upon expulsionfrom a furnace, HCN in gaseous stream 121 is directed to a thermaloxidizer 116. In various embodiments, HCN is directed to a thermaloxidizer via a pipe. For example, pipe system 106 may be used to conductgaseous stream 121 to thermal oxidizer 116. Pipe system 106 may be madefrom any material suitable for this purpose. For example, pipe system106 may be made from metal and may be wholly or partially lined (e.g.,refractory lined).

In various embodiments, various valves and valve like devices may beused to conduct HCN in gaseous stream 121 to thermal oxidizer 116. Forexample, valve 124 and valve 120 may be used to direct the flow ofgaseous stream 121. Valve 124 and valve 120 may be made from anymaterial suitable for this purpose. For example, valve 124 and valve 120may be made from metal and may be wholly or partially lined (e.g.,refractory lined). Valve 120 may be configured to be at least partiallyopen to allow gaseous stream 121 to proceed to thermal oxidizer 116.Valve 124 may be configured to be closed to prevent gaseous stream 121from proceeding to steam vacuum system 112. At times when gaseous stream121 is not present, and thus no HCN is present, valve 124 may beconfigured to be at least partially open.

As noted above, in various embodiments, a thermal oxidizer may beprovided. As used herein, a thermal oxidizer may be any device capableof housing a combustion reaction. For example, a thermal oxidizer maymix air and/or added oxygen to the gaseous HCN and provide an ignitionsource. The combustion of HCN may yield compounds that are less toxicthan HCN, including carbon dioxide, water, nitrous oxides, and otherlike compounds. As used herein, less toxic compounds refer to compoundsthat result from the combustion of HCN. For example, thermal oxidizer116 is in fluid communication with pipe system 106. Thermal oxidizer isconfigured to incinerate or otherwise burn HCN and other materials foremission. Thermal oxidizer 116 may comprise any known or hereinafterdeveloped form of thermal oxidizer. For example, thermal oxidizer 116may be of a direct fired (i.e., afterburner) type, a recuperative type,a regenerative type, a catalytic type and/or a flameless type. Suitablethermal oxidizers are available from John Zink Company, LLC, 11920 EastApache, Tulsa, Okla. USA 74116. In addition, suitable thermal oxidizersare available from Process Combustion Corporation, 5460 Homing Road,Pittsburgh, Pa. USA 15236, MRW Technologies, 1910 West C Street, Jenks,Okla. USA 74037 Inc., EPCON Industrial Systems, 17777 Interstate 45South Conroe, Tex. USA 77385.

In various embodiments, the resulting compounds from combustion in thethermal oxidizer may be forwarded to an environmental mitigation device(e.g., a “scrubber”) or a chimney for emission into the air. Forexample, various environmental mitigation devices may be used to reducethe concentrations of various materials, such as nitrous oxides. Invarious embodiments, the resulting compounds from combustion in thethermal oxidizer are emitted via a chimney. For example, the resultingcompounds from combustion in the thermal oxidizer 116 are released inchimney 114.

With continued reference now to FIG. 1, furnace 108 has inlet 102 andoutlet 104. Furnace 108 also has seal flanges 110 that may be boltedtogether to prevent gaseous emissions. Carbonization may take place asfurnace 108 is heated to above about 1100° C., and in variousembodiments, furnace 108 may be heated to above about 1200° C. Gaseousstream 121 may be formed at temperatures above about 1100° C. as HCN andother cyanides are emitted from the carbon fiber precursors withinfurnace 108. Accordingly, gaseous stream 121 comprises HCN.

Gaseous stream 121 is emitted from furnace 108 at point 109. In variousembodiments, nitrogen gas may be pumped into furnace 108 to assistexpulsion of gaseous stream 121. Pipe system 106 conducts gaseous stream121 away from furnace 108. In various embodiments, pipe system 106comprises multiple pipes, though, in alternate embodiments, pipe system106 comprises a single, continuous pipe. Pipe system 106 may also be atleast partially lined or insulated, and/or at least partially unlined oruninsulated.

In various embodiments and as noted above, valves 124 and 120 arecoupled with pipe system 106. Valve 124 separates pipe system 106 tosteam vacuum system 112. Valve 120 separates pipe system 106 fromthermal oxidizer 116. Valves 124 and 120 may be open or closed, asappropriate, during carbonization and during the second stage. Forexample, valve 120 may be open during carbonization so that gaseousstream 121 will continue to thermal oxidizer 116. Valve 124 may beclosed during carbonization so that the steam vacuum system 112 isprotected from gaseous stream 121 and the HCN contained therein.However, during the second stage, valve 124 may be open so that steamvacuum system may pull a vacuum in furnace 108 and valve 120 may beclosed so that effluent gases during the second stage are not drawn intothe thermal oxidizer. This may be beneficial, as second stage effluentgases may contain residual impurities (such as sodium) that oftenrequire separate material management strategies.

Vacuum system 112 may comprise any suitable system configured to pull avacuum in a furnace 108. For example, vacuum system 112 may comprise asteam vacuum or a mechanical vacuum.

In an embodiment, thermal oxidizer 116 is configured to oxidize gaseousstream 121 and/or the HCN contained therein. Thermal oxidizer 116 mayaccomplish oxidization by combustion of gaseous stream 121, either incombination with air or supplied oxygen.

Now, with reference to FIG. 2, in an embodiment, a method of HCN removal200 comprises heating a furnace to above about 1200° C. in step 201,although step 201 may comprise heating to a temperature in the range offrom above about 1100° C. to below about 1600° C. HCN and/or othercyanides may be expelled from the furnace in step 203. Optionally, aninert gas may be pumped into the furnace to assist in the expulsion ofHCN (not shown in FIG. 2). HCN may be oxidized in step 205 in a thermaloxidizer. The second stage may be performed in step 207. Optionally, avacuum may be drawn using a vacuum device (e.g., pump or steam vacuum)prior to step 207.

Benefits, other advantages, and solutions to problems have beendescribed herein with regard to specific embodiments. However, thebenefits, advantages, solutions to problems, and any elements that maycause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as critical, required, or essentialfeatures or elements of the invention. The scope of the invention isaccordingly to be limited by nothing other than the appended claims, inwhich reference to an element in the singular is not intended to mean“one and only one” unless explicitly so stated, but rather “one ormore.” Moreover, where a phrase similar to “at least one of A, B, or C”is used in the claims, it is intended that the phrase be interpreted tomean that A alone may be present in an embodiment, B alone may bepresent in an embodiment, C alone may be present in an embodiment, orthat any combination of the elements A, B and C may be present in asingle embodiment; for example, A and B, A and C, B and C, or A and Band C. Furthermore, no element, component, or method step in the presentdisclosure is intended to be dedicated to the public regardless ofwhether the element, component, or method step is explicitly recited inthe claims. No claim element herein is to be construed under theprovisions of 35 U.S.C. 112, sixth paragraph, unless the element isexpressly recited using the phrase “means for.” As used herein, theterms “comprises”, “comprising”, or any other variation thereof, areintended to cover a non-exclusive inclusion, such that a process,method, article, or apparatus that comprises a list of elements does notinclude only those elements but may include other elements not expresslylisted or inherent to such process, method, article, or apparatus.

1. A method of transforming a carbon fiber body precursor into a carbonfiber body, comprising: heating a carbon fiber body precursor to atemperature of above about 1200° C. and a pressure of about 1 atm toabout 1.5 atm in a furnace in a first stage; expelling a stream ofeffluent gas outside the furnace, wherein the stream of effluent gascomprises a cyanide; thermally oxidizing the cyanide during the firststage; and heating the carbon fiber body precursor to a temperature ofbetween about 1600° C. and about 2400° C. in a second stage.
 2. Themethod of claim 1, further comprising drawing a vacuum using a steamvacuum prior to entering the second stage.
 3. The method of claim 1,wherein the first stage temperature does not exceed about 1600° C. 4.The method of claim 1, wherein the thermal oxidation occurs in anincinerator.
 5. The method of claim 1, wherein a secondary gas is pumpedinto the furnace during the expelling of the stream of effluent gas. 6.The method of claim 1, wherein the secondary gas is at least one ofnitrogen and argon.
 7. The method of claim 1, wherein the first stagetemperature is between about 1200° C. to about 1300° C.
 8. The method ofclaim 1, wherein the cyanide is hydrogen cyanide.
 9. A system comprisingfor transforming a carbon fiber body precursor into a carbon fiber body,comprising: a furnace having a gas inlet; a thermal oxidizer in fluidcommunication with the furnace; wherein the furnace is capable ofheating a carbon fiber body precursor to above about 1200° C. in a firststage at a pressure of about 1 atm to about 1.5 atm; wherein a stream ofeffluent gas is expelled outside the furnace in response to the furnacereaching a temperature of above about 1200° C.; wherein the stream ofeffluent gas comprises a cyanide; and wherein the thermal oxidizeroxidizes the cyanide during the first stage.
 10. The system of claim 9,further comprising a steam vacuum.
 11. The system of claim 9, whereinthe first stage temperature does not exceed about 1600° C.
 12. Thesystem of claim 9, wherein the thermal oxidizer comprises anincinerator.
 13. The system of claim 9, wherein secondary a gas ispumped into the inlet.
 14. The system of claim 13, wherein the secondarygas is at least one of nitrogen and argon.
 15. The system of claim 9,wherein the first stage temperature is between about 1200° C. to about1300° C.
 16. The system of claim 1, wherein the cyanide is hydrogencyanide.
 17. A method of transforming a carbon fiber body precursor intoa carbon fiber body, comprising: heating the carbon fiber body precursorat a substantially atmospheric pressure to above about 1200° C. in afurnace in a first stage; introducing a secondary gas into the furnaceat a positive pressure; expelling a stream of effluent gas outside thefurnace, wherein the stream of effluent gas comprises a cyanide;thermally oxidizing the cyanide during the first stage using a thermaloxidizer; closing a first valve so that the thermal oxidizer is nolonger in fluid communication with the furnace; opening a second valvein fluid communication with a steam vacuum so that the steam vacuumbecomes in fluid communication with the furnace; and heating the carbonfiber body precursor to a temperature of between about 1600° C. andabout 2200° C. in a second stage.
 18. The method of claim 17, whereinthe thermal oxidizer is an incinerator.
 19. The method of claim 17,wherein the secondary gas is nitrogen gas.
 20. The method of claim 17,further comprising using the steam vacuum to form a vacuum in thefurnace.